1. Field of the Invention
The present invention relates generally to presses and the like having a ram and upper and lower tools for processing workpieces such as sheet metals and more particularly to a method and apparatus for which are applicable to presses in case that the upper tool is mounted free from the ram.
2. Description of the Prior Art
As is well known, presses are provided with a vertically movable ram and upper and lower tools or dies which are worked by the ram to process workpieces such as sheet metals. In such presses, the upper and lower tools are held or mounted generally in two manners so as to cooperate with each other. In one of the manners, the upper tool is secured to the lower end of the ram, while the lower tool is mounted on a work-table or bolster just beneath the ram. In the other manner, both of the upper and lower tools are mounted free from the ram, and both the tools are unitized as a tool assembly or die set to be mounted on the work-table just beneath the ram. Also, in the latter manner, the upper and lower tools are held free from the ram on a tool holding means such as a pair of turret members which are so designed as to hold a number of upper and lower tools and selectively bring a desired pair of upper and lower tools into position just under the ram. Nowadays, the upper and lower tools are mostly employed as a tool assembly or die set for the convenience of installation and for other reasons, and also they are in many cases held on turret members in automatic presses which are numerically controlled.
In the manner in which the upper tool is mounted free from the ram, the upper tool is so arranged as to be drawn up or stripped by a stripping spring out of a workpiece to be processed after each completion of a processing cycle. More particularly, such a stripping spring is so disposed as to be compressed when the ram is urging the upper tool to the workpiece and the lower tool and then lift up or strip the upper tool out of contact with the workpiece.
Especially in punching and blanking operations, however, the upper tool will often fail to be stripped out of the workpiece after a completion of a processing cycle from various causes such as breakage or fatigue of the stripping spring and wear or thermal expansion of the upper tool. Of course, when the upper tool is mis-stripped or not stripped out of the workpiece in punching and blanking operations, it often happens that the upper tool will be caught not only in the workpiece but also in the lower tool. Anyway, it is very dangerous that the upper tool is mis-stripped or fails to be stripped from the workpiece, since the press will go on moving with the upper tool caught in the workpiece. Also, any or all of the upper and lower tools, the workpiece and the press will be damaged or broken if the workpiece is forcedly moved by power when the upper tool is caught in the workpiece because of mis-stripping. Since workpieces are mostly moved or fed into presses automatically by power especially in punching and blanking operations, damage to tools, workpieces and presses has heretofore frequently occurred. Thus, it is absolutely necessary to stop the workpiece from being moved and also the press from being driven the moment the upper tool is caught in the workpiece because of mis-stripping especially when the workpiece is being automatically fed by power. Of course, it is necessary first to detect mis-stripping of the upper tool to stop the workpiece and the press from being moved the moment the upper tool is mis-stripped.
Heretofore, various attempts have been made to detect mis-stripping of upper tools in presses in order to stop workpieces and presses from being moved or driven the moment mis-strippings occur. For example, a photoelectric tube is employed so that it may check each return of the upper tool to its normal position after each completion of a processing cycle so as to stop the workpiece and the press when the upper tool is not normally returned to its position. As another example, a drive source such as an electric motor for automatically feeding the workpiece into the press is so arranged as to be stopped when it is overloaded because of mis-stripping of the upper tools.
However, all of the conventional measures have suffered from serious shortcomings, and it has been impossible to securely detect mis-strippings of upper tools to stop workpieces and presses from moving the moment mis-strippings occur. The manner using the photoelectric tube is not applicable in high-speed punching and blanking operations in which a millisecond matters, since the upper tool takes time to return to its normal position and there will be a time lag between occurrence and detection of mis-stripping of the upper tool. Also, it is difficult to mount the photoelectric tube in tool holding means such as turret members where a number of tools are thickly mounted. In the manner to detect mis-stripping from an overload of the drive source for feeding the workpiece, it takes time to detect the overload of the drive source after the occurrance of mis-stripping, and therefore the workpiece and the press could not be stopped the moment mis-strippings occur in high-speed punching and blanking operations. Furthermore, the drive source will not be overloaded to the extent to be detected when a thin workpiece is being processed even if a mis-stripping of the upper tool occurs.